This invention relates to tunable microwave devices incorporating ferrimagnetic resonance materials such as yttrium-iron garnet (YIG), and more particularly to an electronically tunable YIG band pass filter having a structure for which is easy to acquire identical filter characteristics in the production of the devices with high reliability.
The use of ferrimagnetic devices such as the YIG as electronic tuning elements in microwave band pass filters is relatively well known in the art, for instance, as described in U.S. Pat. No. 3,973,204. FIG. 1 is a schematic view showing a basic circuit configuration employed in a YIG microwave band pass filter. In this illustration three YIG spheres 1a, 1b, 1c are included. Although only one YIG sphere is necessary to construct a band pass filter, a more selective filter may be achieved by using additional YIG spheres.
Coupling loops 2a, 2b, 2c, 2d are disposed around the YIG spheres 1a, 1b, 1c in semicircular form so that they magnetically couple the YIG spheres. Also the loops are positioned orthogonally to each other so that the loops are magnetically decoupled, but the loops are coupled to each other through the resonance of the common YIG spheres. Input signals in FIG. 1 are supplied to the coupling loop 2a and are coupled to the YIG sphere 1a, and through the resonance of the YIG sphere 1a the signals are coupled to the coupling loop 2b.
The signals on the coupling loop 2b are coupled in the same manner, through the resonance of the YIG sphere 1b, to the loop 2c, and the signals at the coupling loop 2c are provided at an output terminal through the resonance of the YIG sphere 1c.
The coupling loops 2a, 2b, 2c, 2d are disposed in grooves 4 provided in a housing 3 as illustrated in FIG. 2. The housing 3 is made of a non-magnetic material like copper or plastic that is metalized on the surface.
In the prior art, these coupling loops 2a, 2b, 2c, 2d are made of thin wire of about 0.1 to 0.2 mm in diameter, or of a small metal film of about 0.1 mm thick and 0.2 to 0.5 mm wide. Since the loops are small and flexible, it is difficult to dispose the loops exactly in the same position and in the same form.
Further these loops are susceptible to shock or vibration and there is a possibility of their being moved or deformed by this shock or vibration. Consequently, variations in filter characteristics arise and thus reliability is low. Also, there is some uncertainty in disposing the coupling loops 2a to 2d in the desired position in the grooves 4 when setting up the band pass filter.
Accordingly, there is variation in the filter characteristics such as insertion loss or passband, and it is difficult to produce band pass filters with identical properties.